This invention relates generally to dynamoelectric machines and more particularly to drive systems involving multiple integrated brushless permanent magnet (PM) motors. PM motors convert electrical energy to kinetic energy by exploiting the electromagnetic relationship between a magnet and an electric field. Conversely, PM generators convert kinetic energy to electrical energy using the inverse of the electromagnetic relationship. PM motors and PM generators are collectively referred to as dynamoelectric machines. In a typical PM motor, electric current is passed through stationary windings of conductive wires to generate an alternating magnetic field to push and/or pull a magnetic rotor. The magnetic rotor is coupled to a shaft to produce rotational shaft power. Additionally, high output torque can be obtained from PM motors at low rotor speeds. For these reasons and others, PM motors are well suited for propulsion systems for large, track-laying vehicles such as for military vehicles or construction equipment. Variable field (VF) PM motors are particularly suited for propulsion systems for electric vehicles because of their ability to operate in a constant power mode beyond their conventional corner point, the point of maximum speed output for the given rotor/stator alignment. VFPM motors adjust the relative axial position of the magnet and the conductive wires to adjust the magnetic flux interaction of the two components. Thus, VFPM motors typically require additional axial length for the motor housing such that the magnet and conductive wires can be drawn apart.
Cross-drive type propulsion systems using PM motors are popular selections for electrically powered track laying vehicles due to their ability to transfer power from one vehicle track to the other. However, in order to do so, cross-drive systems typically require the use of a plurality of electric motors. For example, one PM motor is required to provide vehicle propulsion and another motor is required to provide steering power. Thus, cross-drive propulsion systems are typically cumbersome, as the PM motors must be stacked either axially or radially. However, in vehicle propulsion systems, as well as for other diverse ranging applications, it is desirable to reduce space consumption of the PM motors to make space available for other uses, such as for cargo. It is also desirable to keep the width of electric vehicles compatible with roadways and freighting equipment, such as flatbed trucks and trains. Therefore, there is a need for a more compact dynamoelectric machine, and in particular, for a more compact dual variable field permanent magnet motor.